1. Field of the Invention
The present invention relates to an apparatus and method of receiving multi-path signals in a direct sequence CDMA mobile communication system.
2. Description of the Related Art
In light of the rapid increasing trend of wireless communication services, a saturation phenomenon of the wireless propagation spectrums can be easily anticipated. Accordingly, it is necessary to develop a new wireless communication technique having a superior frequency efficiency. A representative example of such a wireless communication technique may be a code division multiple access (CDMA) system.
In the CDMA system, a wide frequency band is commonly and simultaneously used by many users. That is, respective users simultaneously transmit signals modulated to wide bands using a diffusion band method, and detect a signal transmitted from a desired person using respective codes (or sequences). In the mobile communication system using the CDMA system, the transmitted data is not easily exposed, and a high-grade security can be achieved in comparison to other multi-connection systems. The CDMA system is divided into a direct sequence CDMA (DS/CDMA) system, a frequency hopping CDMA (FH/CDMA) system, etc., in accordance with a frequency diffusing method.
The DS/CDMA system diffuses a signal spectrum area of a signal to be transmitted by coding the signal using a user's inherent pseudo-noise (PN) sequence, and converts the signal into a wideband signal. In the DS/CDMA system, a signal transmission through a multi-path is typically performed. In the DS/CDMA mobile communication system, a multi-path receiver (hereinafter referred to as a “rake receiver”) demodulates the multi-path signals received through different paths, and has a time diversity effect. For this, the rake receiver has a plurality of fingers. The respective fingers are allocated with the multi-path signals having different time delays through respective paths, and signals processed through the respective fingers are combined to heighten the receiving quality.
FIG. 1 is a block diagram illustrating an example of the construction of a conventional rake receiver used in the DS/CDMA mobile communication system.
Referring to FIG. 1, the rake receiver includes a searcher 120, a plurality of fingers 130, 140 and 150, a controller 110, and a combiner 160. The controller 110 manages the searcher 120, the fingers 130, 140 and 150, and the combiner 160. The searcher 120 detects power levels and position information of the multi-path signals. The respective fingers 130, 140 and 150 are allocated with a specified multi-path among the multi-paths from the controller 110, and each tracks its path and demodulates the multi-path signal received through the specified multi-path. The respective fingers 130, 140 and 150 include samplers 132, 142 and 152, code trackers 134, 144 and 154, and demodulators 136, 146 and 156, respectively. The combiner 160 receives demodulated symbol strings from the plurality of fingers 130, 140 and 150, and combines them into a single demodulated symbol string to output the single demodulated symbol string.
The operation of the rake receiver as constructed above will be explained. The controller 110 transfers a multi-path search command to the searcher 120. The searcher 120, which received the multi-path search command, measures the power levels of the multi-path signals. Then, the searcher 120 reports the position information of the multi-path signals to the controller 110 along with the measured power levels of the multi-path signals. The controller 110 receives the power levels and the position information of the multi-path signals from the searcher 120, and determines the multi-path signals required to be demodulated in accordance with the power levels and the position information. Then, the controller 110 allocates the fingers to the multi-path signals required to be demodulated, and outputs a demodulation command for requesting the demodulation of the corresponding multi-path signals to the allocated fingers. It is assumed that the allocation is performed with respect to the fingers 130, 140 and 150. The fingers 130, 140 and 150, which received the demodulation command, track the multi-path signals allocated to themselves, and demodulate the tracked multi-path signals. The demodulated symbol strings demodulated by the fingers 130, 140 and 150 are transferred to the combiner 160. Meanwhile, the fingers 130, 140 and 150 measure qualities of the multi-path signals which are now being demodulated, and report the measured qualities to the controller 110. The controller 110 determines whether to continue the demodulation of the specified multi-path signal using the qualities of the multi-path signals reported from the fingers 130, 140 and 150 and the power levels and the position information of the multi-path signals reported from the searcher 120. The combiner 160 combines the demodulated symbol strings transferred from the fingers 130, 140 and 150, and outputs a single demodulated symbol string resulting from the combining.
Hereinafter, the operation of the fingers which constitute the rake receiver will be explained in detail.
The code trackers 134, 144, and 154 search for the optimum sample positions so as to maximize the signal powers of the multi-path signals in synchronization with the multi-path signals. The optimum sample positions searched by the code trackers 134, 144 and 154 are provided to the samplers 132, 142 and 152 and the code trackers 134, 144 and 154, respectively, as timing control signals. The samplers 132, 142 and 152 sample the corresponding multi-path signals in accordance with the timing control signals from the code trackers 134, 144 and 154, and provide the sampled multi-path signals to the demodulators 136, 146 and 156 and the code trackers 134, 144 and 154. The demodulators 136, 146 and 156 demodulate the multi-path signals from the samplers 132, 142 and 152 using samples of the optimum sample positions detected by the code trackers 134, 144 and 154.
The code trackers 134, 144 and 154 search for the optimum sample positions using search windows of a predetermined size that corresponds to one sample. The search window can be expressed as a distance between the position of a late phase (hereinafter referred to as a “late hypothesis”) and the position of an early phase (hereinafter referred to as an “early hypothesis”). The code trackers 134, 144 and 154 search for the optimum sample positions by moving the sample positions in a direction that a correlation value becomes larger using the search window, i.e., the difference between a correlation value of the late hypothesis and a correlation value of the early hypothesis. Generally, the time point where the difference between the correlation value of the late hypothesis and the correlation value of the early hypothesis is “0” corresponds to the optimum sample position. In performing the code tracking, the interval between the center axis of the search window and the late hypothesis or the interval between the center axis of the search window and the early hypothesis is set to be within one chip, and generally to be 0.5 chip. The converging positions of the code trackers 134, 144 and 154 depend upon power delay profiles of the multi-path signals.
In the multi-paths, as the time delay between the adjacent paths becomes greater and the power difference between the adjacent paths becomes smaller, the convergence of the respective path becomes easier. However, as the time delay between the adjacent paths gets smaller and the power difference between the adjacent paths becomes greater, the multi-path appears to be one path, and this causes the probability that the code trackers corresponding to the respective fingers converge to the same position to be increased. Several fingers tracking the same path is called a “fat finger phenomenon”.
As described above, the respective fingers 130, 140 and 150 that constitute the rake receiver are allocated with the multi-paths having different time delays from the controller 110, and let the code trackers 134, 144 and 154 continuously track the multi-path signals received through the allocated multi-paths. The operation of the code tracker, the construction of which is illustrated in FIG. 2, will be explained in detail.
Referring to FIG. 2, an early-hypothesis correlator 210 obtains a sample position that is earlier than the position of a sample input to the demodulators, i.e., an early-hypothesis correlation, and outputs a first correlation value according to the correlation. A late-hypothesis correlator 212 obtains a sample position that is later than the position of a sample input to the demodulators, i.e., a late-hypothesis correlation, and outputs a second correlation value according to the correlation. The first correlation value and the second correlation value are subtracted by a subtracter 214, and a timing error is detected by the correlation difference according to the subtraction. The timing error passes through a loop filter 216 to be output as a timing control signal for obtaining the optimum sample position. The optimum sample position as obtained above depends upon an envelop of a power delay profile of a normal multi-path signal, and has a tendency to converge to a peak point of the envelop.
FIG. 3 is a graph illustrating an example of a power delay profile in the multi-path environment. In FIG. 3, paths L0 and L1, which have different time delays, have a time interval that is as long as Td. It is assumed that under such a power delay profile, the paths L0 and L1 are allocated to different fingers, and Td is larger than one chip. Since the early-hypothesis correlation and the late-hypothesis correlation are not affected by other paths and thus are not greatly affected by noise, the code trackers of the respective fingers can continuously keep the synchronization with the respective paths.
FIG. 4 is a graph illustrating another example of a power delay profile in the multi-path environment. In FIG. 4, it is assumed that Td is smaller than one chip or the power difference between the adjacent paths is great. In this case, in the code tracker of the second finger which is allocated with the path L1, the early-hypothesis correlation value becomes greater than the late-hypothesis correlation value. Accordingly, the timing control signal commands to move the search window in a direction where the correlation is great, i.e., in the direction of the path L0. Through this operation, the two fingers, which were first allocated to the paths having the different time delays, converge to the path L0 after a predetermined time elapses. This means that the fat finger phenomenon occurs, in which two different fingers track the same path.
The fat finger phenomenon occurring as above deteriorates the receiving performance of the rake receiver, the purpose of which is to obtain the time diversity effect by combining components of the multi-path signals having different delays.
As an example of the multi-path environment having the power delay profile as shown in FIG. 4, a case-3 channel may be cited among channel conditions presented in the technical specification (TS) that describes a terminal request performance with respect to a wideband CDMA (WCDMA) proposed in the 3rd generation partnership project (3GPP). An impulse response of the case-3 channel has the characteristics as shown in Table 1 below. For reference, the period of one chip in the WCDMA system is 1/3840000 sec (about 260 ns).
TABLE 1RelativeMeanDelay (ns)Power (dB)00260−3521−6781−9
In the WCDMA system, a square raised cosine (SRRC) filter is used as a pulse shaping filter of the transmitting end and the receiving end. The impulse response characteristic according to the pulse shaping filter is shown in FIG. 5. Accordingly, an envelop |γ(t)| of the impulse response r(t) of the signal received in a modem through the case-3 channel proposed in the TS and the pulse shaping filter is shown in FIG. 6. The envelop of FIG. 6 has the same characteristic as that of FIG. 4. Due to the above-described characteristics, the fat finger phenomenon occurs in the conventional multi-path receiver. In this case, the receiving performance request condition proposed in the 3GPP cannot be satisfied.